Retractable undercarriage for aircraft



Dec. 19, 1950, c. H. FLURSCHEIM RETRACTABLE UNDERCARRIAGE FOR AIRCRAFT 3 Sheets-Sheet 1 Filed May 30, 1945 Fig. 2.

Inventor-z Cedric H. Fl urscheim,

by Mail/ H is Attorney C. H. FLURSCHEIM RETRACTABLE UNDERCARRIAGE FOR AIRCRAFT Dec. 19, 1950 3 Sheets-Sheet 2 Filed May 30, 1945 Imvenfor V Cedric HhF lu -scheim, by 22; W

Hrs Attorney 3 Sheets-Sheet 3 U/Il/Ill/l/lll/lll/l/ IIll/l/l/l/lY/lllIllII/Il/llIll Dec. 19, 1950 c. H. FLURSCHEIM RETRACTABLE UNDERCARRIAGE FOR AIRCRAFT Filed May so, 1945 Fig.4.

46 Inventor: Cedric, H. Flursgheim,

-' His Attorney Patented Dec. 19, 1950 ETRAC ABLE U BCABRIAG R AIRCRAFT Cedric Harold Flurscheim, Bowdon England, as-

signor, by mesneia'ssignments, to GBIlBlfll-EIBO? tric Company, a corporation of New York pIicationMay 30, 1945, Serial No.596,737

In Great BritainOctoher 16, 1941 S ction 1, HhIi'Q 690, Au ust 1 6 a n EXB F to e 16 1 1 This invention relates to retractable undercarriages for aircraft.

The present invention provides an improved retractable undercarriage for aircraft comprising a pivotal leg for carrying the landing wheel or equivalent landing element, a pair of teggle links for bracing the landing-element-carrying leg in the extended position, an improved term of relatively movable latch mechanism fencer.- ing the toggle links in their made, or Straightened, position and providedwith improved oriented and limited releasing means such as to enable a chain or cable tobe interconnected through the agency of said releasing means of th improved latch mechanism with the links and, passing over a reversible winch for sequentially e'ifectingrer lease operation of the latch mechanism prior to unstraightening the toggle and then applying tensile force to one of the toggle links for effect.- ing retraction of the undercarriage,

A pair of toggle links for bracing the landingelement-carrying legmay conveniently comprise a relatively long linl; pivoted .tothe aircraft,

namely to the Win nacelle or fusel ee as the 125 case may be, correspondingly with the landing,- .elernent-carrying leg, and a relatively much shorter link pivoted to the landingeeleznentecarr inglee, the chain or a e b nscen en e tly co c by means o he la ch m chani m to one of the toggle links so as to apply force for first releasing the latch mechanism prior to breaking the toggle and retracting the undercarriage at .or adjacent to the knee joint between the links from a point substantially vertically above said joint in the straightened position. of the links.

Preferably the improved latch mechanism is-of the eccentric relatively rotatable roller interlocking type and is normally biased, {for example by spring means, to the position for locking the toggle links in their straightened position so as to readily snap into interlocking engagement immediately upon the straightening of thet'oggle links.

Also, it is preferred to connect the chain vvo1 cable to .the toggle links through the int'ermee diary of the latch releasing mechanism such manner as tooperate the latch iornnloclgingthe toggle prior to applying tensile force for'breaking the toggle and then retracting the underrr'i sa f It is customary to provide a leg or equivalent "for carrying the landing wheel or like element in 'tance fo taking upload when the wheeler like landing element makes contactwith the landing surface. In such eq-llipment'it has already been proposed to utilise the shock a'bsorberas an energy storing deyice acting to accumulate energy duringthe first part of the'retraction movement Q t und rca r a an o e ndsu h ene duri t ema n ng Pa o t t a it movement, namely during which the weight of the retractable parts necessitates the application of greater mechanical efiort than that reqnired duringthe aerodynamically-assisted first part or the retraction movement.

In the improved undercarriage of this inyen tier; it is preferred to employ v a'lancling{elemant 'in leg ont i i a l t i B tlet a s: silient shock absorber, for example of the known telescopically resili nt t p tor resilien l sun'- porting thelanding element, that part dftherLe;

' si1ient'shock absorber which is resilientlym oy able, i h the n in e m n r l ive o the am ent?a r ia lea o th a n pa t -hereet be n e t t a i L'Q wi le links in such manner as to apply a tor qne for bia ng t o l 1 i into t r st ai tened position in which they are adaptedto he'locked b he atq n a n et rr d r a e nt o of the ale links o b a ng t and n -ze m nt a r ns s P votal h l a or a least to the part olflthe shock absorher which is sta; t ioner-y with the leg, or the main part thereof, relatively to the resiliently sprung 'landi A m atand is teta a l bout e r e m retraction of the leg so that, during a a l o ns le t m by e ha or cabl t he tes links, the angleofinclination ofsai'cl toggle'linl:

with the leg is reduced and thereafter increased and that part of the shock absorber which is movable, with the landing element, relative to the leg, or main part thereof, is connected to said pivoted toggle link, for example by linkage to a point intermediate the pivot between the toggle link and the leg on the one hand and the joint between the toggle links on the other hand, the arrangement-being such that energy is accumulated in the shock absorber during the initial part of the retraction movement, namely during reduction of said angle of inclination of the toggle link with the leg, but is expended by the shock absorber in the final part of the retraction movement, namely during increase of said angle of inclination of the toggle link .with the leg, for assisting in effecting the retraction movement, and

conversely, energy is accumulated in the shock absorber during the initial part, namely whilst the angle of inclination is being reduced, but is expended .by the shock absorber during the final part of the extension movement, namely during I increase of said angle of inclination, for assisting in rotating the said toggle link into the position in which the toggle links will be locked by the latch. The resiliently sprung part may be connected to one of the toggle links, either directly or through a linkage, so as to enable lost motion between these parts, one of the toggle links being pivotally connected, preferably directly, to the landing-element-carrying leg, 01- to the part of the shock absorber from which the landing element is adapted to be sprung. Conveniently, the

resiliently sprung part may be connected to one of the toggle links by a lost motion device in the form of a pin and slotted link. In a preferred form, however, the lost motion connection comprises a lever movable, through the medium of a link, with the resiliently sprung wheel or like landing element assembly about a fixed pivot and constrained to bear on a projection or roller carried by the said toggle link.

The aforesaid latch may comprise a bell crank lever pivoted to one of the toggle links at a point adjacent the joint with the other toggle link and provided on one arm with a roller adapted to engage with a bearing surface on the other link, whilst its other arm is connected to spring means tending to cause the aforesaid engagement and thereby lock the toggle links in their straightened position. For effecting release of the latch, the aforesaid cable or chain will be conveniently connected to the roller-carrying arm of said lever.

In another form, the latch may comprise a plate pivoted about the axis of the joint between the toggle links and having pivoted to it a springpressed substantially tangential link carrying a knuckle-jointed extension which is pivoted to one of the toggle links so as to be capable of embracing a roller carried at, or adjacent, the end of the other link of the toggle. In this form, release of the latch is effected by rotation of the aforesaid plate, as may conveniently be effected by a pull transmitted through the aforesaid cable or chain, which will be appropriately connected to it for this purpose.

If desired, the extension movement of the undercarriage may be assisted by thrust applied to the toggle links through the cable or chain in the manner disclosed in U. S. Patent 2,446,415.

In order that the invention may be fully understood reference will now be made to the accompanying somewhat diagrammatic drawings, in which:

Figure 1 is a side elevation of a retractable un- 4 dercarriage arranged in accordance with the invention, the undercarriage being shown, in full line, in the fully extended position and, in chaindotted line, in the fully retracted position,

Fig. 1a shows to different scale the means for latching the pair of toggle links of Fig. l in their straightened position,

Fig. 2 shows typical load curves during retraction of the undercarriage,

Fig. 3 is a side elevation, to enlarged scale, of another arrangement, according to the invention, for connecting the landing-element-cariying leg and the bracing toggle linkage, alternative to the form of such connection indicated in Fig. 1.

Fig. 4 is a sectional view, to enlarged scale, of another form of latching means for the toggle linkage alternative to that shown in Fig. l, and

Figs. 5 and 6 are respectively side elevational and plan views of a still further modified form of latching means for the toggle linkage.

Referring first to Fig. 1, there is indicated a wheel-carrying leg I pivoted at joint 2 to the wing, nacelle or fuselage of the aircraft represented generally at 3, so that the leg is retractable by swinging backwards about the pivot 2. The leg I is adapted to be braced in the extended position shown, by a toggle strut comprising a short toggle link 4 and a relatively much longer toggle link 5 pivoted together at 6, the shorter link 4 being further pivoted at 4a to the leg I and the longer link 5 at 50 to the wing, nacelle or fuselage 3, to the rear of, and in substantially horizontal alignment with, the pivot 2 of the leg I.

The leg I is in the known form of telescopically resilient shock absorber comprising a tubular part 'I and a plunger or equivalent member 8 telescoping within the tube I and subjected, Within said tubular part, to the action of high pressure oil or other damping fluid which is permanently sealed within the tube 1. The telescoping part 8 extends from the tubular part I through a hermetic sealing and dust-preventing means represented generally at 9 and its protruding end is arranged to support the axle ID of the landing wheel II. The travel of the plunger 8 will be limited by positive stops (not shown), the permitted movement of said plunger enabling the landing wheel I I to move a few inches in the axial direction of the leg I as the load is taken up by the wheel II at contact with the ground or other landing surface.

It will be appreciated that the leg i and bracing toggle strut 4, 5 may form the only support for the landing wheel II' or there may be two such legs and bracing struts astride the wheel, according to circumstances. Likewise, there may be one such leg and bracing strut or pair thereof for each landing wheel or pair of landing wheels. In other words, the leg I, bracing strut 4, 5 and landing wheel I I may constitute the aircraft undercarriage or co-operate with one or more sets of corresponding parts to constitute the undercarriage, according to circumstances.

The plunger 8, at the part where it protrudes from the tube 1, has pivoted to it a link I2 having a longitudinal slot I3 in which engages a pin I4 secured to the shorter toggle link 4. In the fully extended position shown, the pin I l rests at the upper end of the slot I3. However, on contact between the wheel II and the landing surface, the wheel II and plunger 8 will move upwardly against the action of the pressure fluid in the tube 1, the pin I4 riding along the slot I3 into an intermediate position therein.

As shown more clearly in Fig. 1a, the toggle curved surfaces I? projecting in the axial direction of the part 56 for engaging a roller I8 secured to the free end of; the longer toggle link 5. The latch i is biased into the latching .position shown, namely in which the roller I8 is disposed between the surfaces Il' by means of biasing spring it extending between a pin I5a on the latch lever i5 and a pin ib on the toggle link 4, said latch being held against stop to by means of spring i9;

For the purpose of unlatching the latch 15 and applying force to break the toggle strut 5 and to retract the undercarriage, a cable or "chain member 20 passing round a roller 6a on pivot 6 at the knee of the toggle 'is carried upwardly over, a sprocket 2i pivotally mounted to the wing or like part 3, substantially vertically above the toggle pivot 6, the tail of the cable or chain 20 being then accommodated in a guide 22, suitably secured to the wing or like part 3. The sp'rocketZI is adapted to be rotated by means (not shown) such for example as an electric motor. In this instance, the cable or like part 20', after passing round the roller 60. at the toggle pivot 6, is attached to pin 15a on the latch I5. a

With the arrangement described, when the sprocket 2I is driven in the appropriate direction, the initial effect of the pull transmitted through the cable or chain 20 is to rotate the latch I5 against the bias of spring i9 into position to flee from the surfaces H the toggle carried roller I8, Thereafter the continued pull is efiective' to breakthe toggle linkage 4, 5 and, with the toggle folding-up, swing the landing wheel leg l and landing wheel II rearwards and upwards into the stowed or fully retracted position shown in chain-dotted line, in which position the undercarriage will be held by suitable locking of the sprocket 2| against rotation. When the sprocket is released and allowed to rotate in the'opposite direction, or when driven in such opposite direction, the toggle linkage 4-, 5 will unfold and the landing leg I and landing wheel Ii will swing downwardly and forwardly into the position shown, with over-travel of link 5 prevented by abutment 4d.

The slotted link I2, I3 and the pin I 4 enables lost motion between the plunger 8, and thus the landing wheel I I, on the one hand and the-toga gle link 4 on the other hand, for enabling their relative movement under landing loads.

Durin retraction of the undercarriage caused, as described, by the exertion of a pull on the knee joint 6 of the toggle linkage 4, 5, the angle 0 between the toggle link 4 and the wheel-carrying leg I is continuously reduced and, after. the lost motion between these parts has been taken up, namely while the pin I4 is riding along the slot I3 from the intermediate position which it occupies with the landing wheel II in con tact with the landing surface to the end position shown, which it occupies when the landing wheel is .just out of contact with the landing surface, the plunger 8 is accordingly constrained to telescope further into the tube I against the action of the pressure fluid therein. Such movement of the,

- plunger 8 produces corresponding increase in the pressure of the fluid trapped between the plunger 8 and the tube 1. Thus, during this part 0f the retraction movement, energy is stored up i the t s al y res li shock. absorber 00 knee joint 5 of the figure will result in the tog gle link 4 swinging about its pivot 4a in the op?- posite direction, that is, the angle 0 will 00!? responding'ly increase until the undercarriage arrives in the stowed-away position indicated'in chain-dotted line; during this, the final part, of the retraction of the undercarriage, the plunger '8, under the constraint' imposed on it by the high fluid pressure in the tube 'I, moves out!- wardly from the tube. That is to say, whereas energy is stored in the telescopically resilient shock absorberl during the initial part of the retraction of the undercarriage, such stored on; ergyis expended by the shock absorber during the final part of the retraction movement. This expenditure of energy during the final part of the retraction movement is effectual to assist in the retraction movement and thus to reduce the load on the driving means for the sprocket 2|, a feature which is especiallydesirable since, during this part of the retractionjg-reater load would otherwise'be imposed on the driving means by reason of the reduced aerodynamic efiect tending to retract the undercarriage, which re duction is occasioned in consequence oi the re duction in the efiective area of the undercarriage which'is exposed.

Conversely, during the initial part of the e'xtension movement of the undercarriage, the rela't'i've movement of the toggle linkage '4, 5 and the wheel-carrying leg I, namely with angle!) decreasing, will have the efiect of forcing the plunger 8 further into the tube I and so increasing the pressure of the trapped fluid. When the toggle link 4 swings into the position in which it is substantially in alignment with the leg I, that' is with the angle 6 at its minimum value, the telescopic movement of the plunger 8 ceases and "thereafter the plunger 8 is forced by the trapped pressure fluid to move further out from the tube 1. Thus, whereas energy is accumulated in the telescopically resilient shock. absorber I during the initial part of the extension of the undercarriage, namely with the angle 6 decreasing, this energy is expended by the shock absorber during the final part of the extension movement, namely with the angle '0 increasing. As will be appreciated, by virtue of the energy accumulated in the shock absorber during the initial part of the extension movement, torque is applied through the link I2 to the toggle link 4 during the final part of the extension'movement, such torque being in the direction tending to restore the toggle 4, 5 into the straightened position in which it will become locked by the latch I5. Thus, the latching torque acting on the toggle is not only maintained but is actually increased as the extension of the undercarriage progresses.

The operation is further illustrated by reference to the typical load curves shown in Fig. 2 for aspecific example in which the landing wheel weight is of the order of 500 pounds and the 'forceex l erted' by the wheel struts in the extended eondi tionis ofth'e order of 17,500 pounds Weight;

-7 this figure, abscissa represent extent of travel of the lifting cable in inches, this value being zero when the landing wheel is in th fully stowedaway or retracted position, and maximum with the wheel inthe fully lowered position, corresponding to the undercarriage being fully extended, whilst ordinates represent value of pull in pounds weight exerted in the lifting cable during movement of the landing wheel from the fully lowered to the fully raised position. The curve A represents the actual load, that is the force exerted in the cable, throughout the Wheel raising operation, whilst the curve B represents the force required in the cable merely to overcome the weight of the wheel, 50 that the area between the curves A and B, for the higher values of travel of the lifting cable, represents the work done in compressing the telescopically resilient shock absorber whilst the area between these curves for the lower values of travel of the lifting cable represents the work done in effectin extension of the undercarriage by expansion of the telescopically resilient shock absorber.

In the modification shown in Fig. 3, in which parts corresponding to those appearing in Fig; l are referred to with like reference characters, the wheel-carrying leg I is shown as comprising a tubular part 1 and a telescoping part 8. which, as in the previously described arrangement, will be subjected within the tube 1 to the action of pressure fluid. At I is indicated the landing wheel axle, being herein shown as mounted in a bearing comprising a half-block 3a attached to the telescoping part8 and half-block 81) connected to the half-block 8 1 by, means of studs 80. As in the previously described arrangement, the wheel axle I0 is thus movable with the telescoping part 8' relatively to the tube 1, and the landing wheel is accordingly resiliently sprung by the telescopically resilient shock absorber constituted by the wheel-carrying leg I. In this embodiment, however, the wheel axle I0 rides in a slotted link 24 which extends side by side with the leg I and carries, at its upper end, an intermediate pivot 24a for a lever 241) which is also pivoted at 240 to .the tubular part 1. The other end of the lever 24b rests in contact with a roller 4e carried bythe shorter togglelink 4 which is pivoted to the tube 1 at 4a and is adapted to brace the leg I in the extended positionas in the embodiment previously described withreference to Fig. l.

With the modified arrangement of Fig. 3, the lever 24b is constrained to bear upon the roller 46 in the fully extended position of the wheelcarrying leg'I shown, by reason of the action of the trapped pressure fluid on the part 8 and thus the wheel axle I0. However, during movement of the toggle link 4 in the direction of collapse of the toggle, as represented in chain-dotted line; the lever 24b is tilted upwardly drawing the wheel axle I0 into the chain-dotted position'ac- -cordingly to cause the part 8 to telescope into the tube 1 and further to compress the trapped fluid. Conversely, when the toggle link 4 swings in thr direction for making the toggle, the pressure fluid, in forcing the part 8 further out of the tube 1, is enabled, through the resultant displacement of lever 24b, to exert torque on the toggle link 4 in the direction tending to latch the toggle.

According to the modification shown'in Fig. 4, the toggle strut for bracing the wheel-carrying leg I of the undercarriage of Fig. 1 comprises two" links in the form of tubes 34 and 35, corresponding respectively to the short and longer toggle links 4 and of Fig. 1, having a pivoted joint 36 carried by housing 31 for the tube 34. In order to latch the tubes 34 and in the position shown, which is that for the fully extended position of the undercarriage, there is provided a latch in the form of bell-crank lever 38 pivoted at 39 to the longer toggle link 35 and carrying, on one of its arms, a roller 40 for engaging a cam surface 4I on the housing 31 fast to the toggle link tube 34. The latch is biased into the latching position shown by means of spring 42 secured, at one end, to the other arm of the bell-crank latching lever 38 and arranged to be secured, at its other end, to the longer toggle link tube 35. In this embodiment, the latching lever 39 is adapted to be rocked to carry the roller 40 away from the cam 4I so as to release the latch, and the toggle link thereafter broken for retracting the undercarriage by pull exerted in a cable or chain 20, corresponding to the cable or chain 20 of Fig. 1. In other respects the arrangement is similar to that shown in Fig. 1.

In the modification shown in Figs. 5 and 6, the toggle strut comprises two links, one in the form of a pair of bars 44a and 4417 each of elliptical cross-section braced by cross member 440, and the other in the form of a pair of bars 45a and 45b of like elliptical cross-section braced by crossmember 450, the former corresponding to the short link 4 and the latter to the longer link 5 of Fig. 1. The shorter link 44ac is pivoted to the longer link 4511-0 by means of pivot pin 48, the pivot being adjacent the end of the short link but some distance from the end of the longer link. A plate 41 rides on the pivot pin 46, midway between the bars forming the respective links 4411-0 and 45a--c. Pivoted to the plate 41 at 48 is a substantially tangentially disposed link 49. This link 49 is pivoted at 50 to a knucklejointed extension 5| which is also pivoted at 52 to the short toggle link 44a-c. The link 49 is also connected at 53 to one end of a spring 54 whose other end is pivoted at 55 to the plate 41, so that the link 49 is biased into the position shown in which the knuckle-jointed extension 5| substantially embraces a roller 56 carried on the free end of the longer toggle link 45ac, corresponding to the toggle being made and latched, and the undercarriage thus locked in the fully extended position. A cable or chain 51, corresponding to the like part in Fig. 1 is secured to the plate at 58 namely in position such that, when a pull is exerted on the cable Or chain 51, the plate 41 is rotated in the direction to swing the knuckle-jointed extension about its pivot 52 against the biasing action of spring 54 for disengaging the roller 56 and thereafter breaking the toggle for retracting the undercarriage.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A retractable undercarriage for aircraft having in combination a landing wheel having a pivoted supporting leg for carrying said landing wheel between a retracted position and an extended position, means including a pair of toggle links connected by a knee joint and having pivotal connections at their extremities to said leg and to an aircraft respectively to brace said leg against that aircraft when extended to a straightened position, said links having relatively movable interlock members carried thereby into interlocking engagement for locking said links in said straightened position, oriented releasing means including a winch having a flexible tensile element connected with one of said interlock of said toggle links to unstraighten said toggle.

links and pull said leg and said landing wheel to the completely retracted position.

2. A retractable undercarriage according to claim 1 wherein said toggle links for bracing said leg comprise a relatively long link provided with pivotal connections adapted for pivoting said long link to an aircraft body and arelatively short link-pivotedly connected to said leg, said tensile element having means for providing a concentric connection with the axis of the knee joint of said toggle links to apply tensile force to said knee joint from a point substantially vertically above said joint.

3. A retractable undercarriage according to claim 1 wherein said one of said relatively movable interlocking members is provided with spring biasing means for biasing said member to a predetermined locking position in the path of the other member for automatically interlocking therewith with a snap action whenever said toggle links move to said straightened position.

4. A retractable undercarriage according to claim 1 wherein said relatively movable interlock members are disposed remotely from the knee joint between said toggle links and provided with unlocking means including a rotatable member concentric with said knee joint, and said tensile element is connected with said rotatable memher for eifecting an unlocking limited relative rotation thereof for unlocking said interlock members prior to applying tensile force to said knee joint to unstraighten said links and pull said leg and said landing wheel to said retracted position.

5. A retractable undercarriage for aircraft having in combination a landing wheel assembly, a pivoted supporting leg for carrying said landing wheel assembly between a retracted position and a downwardly extended position, a resilient shock absorber for mounting said landing wheel assembly at the free end of said leg for downwardly biased movement relative to said leg, means in- 4 cluding a pair of toggle links connected by a knee joint and having pivotal connections at their extremities to said leg and to an aircraft respectively to brace said leg against that aircraft when extended to a straightened position, one of said links having a connection to said landing wheel assembly for biasing said toggle links toward said straightened position upon downward movement of said wheel, said links having relatively movable interlock members carried thereby into interlocking engagement for locking said links in said straightened position, and means including a winch having a flexible tensile element connected with one of said interlock members for effecting an unlocking limited relative movement thereof, and thereafter applying tensile force to one of said toggle links to unstraighten said toggle links and pull said leg and said landing wheel to said retracted position against the bias of said shock absorber.

6. A retractable undercarriage according to claim 5 wherein said one of said toggle links for bracing said landing wheel carrying leg is pivoted to said leg so that during application of tensile force by said tensile element, the angle of inclination between said one of said toggle links and said leg is progressively reduced and then increased whereby energy is accumulated in said shock absorber during the initial part of :said retraction movement and is expended by said shock absorber in the final part of said retro-- action movement and conversely energy is ac-} cumulated in said shock absorber in the initial. part of the extension .movement of said leg and;

expended in the final part of said extension movement for assisting in rotating said toggle links;

into said straightened position thereof in which said toggle links are locked by said interlocke members.

7. A retractable undercarriage claim 5 wherein said connection between said one link and said shock absorber ;is of the lost motion type to provide freedom of movement of said wheel when said wheel is in the extended position.

8. A retractable undercarriage according to claim 5 wherein said connection is of the pin and slotted link type for providing freedom of movement of said wheel when said wheel is in the extended position.

9. A retractable undercarriage according to claim 5 wherein said one link carries a roller and said connection further comprises a cam lever pivotally mounted on said leg and means for connecting said cam lever to said shock absorber in such a manner that during transition of the leg from the retracted to the extended position, said roller biases said cam lever against the resistance of said shock absorber.

10. A retractable undercarriage for aircraft having in combination a landing wheel having a pivoted supporting leg for carrying said landing wheel between a retracted position and an extended position, means including a pair of toggle links connected by a laterally offset knee joint and having pivotal connections at their extremities to said leg and to an aircraft respectively to brace said leg against that aircraft when extended to a straightened position, one of said links having a bell-crank lever pivoted thereto at a point opposite said knee joint and provided on one arm with a roller the other of said links having a curved bearing surface provided thereon to be engaged by said roller upon straightening of said links, spring means connected with the other arm of said bell-crank for biasing said roller to a position for engaging said surface and thereby interlocking said roller with said surface to lock said links in said straightening position, and means including a winch having a flexible tensile element connected with said bell-crank lever for effecting an unlocking limited relative rotation thereof against the bias of said spring and thereafter applying tensile force to one of said toggle links to unstraighten said toggle links and pull said leg and said landing wheel to said retracted position,

11. A retractable undercarriage for aircraft having in combination a landing wheel having a pivoted supporting leg for carrying said landing wheel between a retracted position and an extended position, means including a pair of toggle links connected by a knee joint and having pivotal connections at their extremities to said leg and to an aircraft respectively to brace said leg against that aircraft when extended to a straightened position, a plate rotatable about the axis of the knee joint of said toggle links, one of said links having an extension beyond said axis and provided with a roller at the end of said extension, a knuckling extension pivoted to the other of said toggle links and provided with a curved sur-v ace for embracing said roller when said toggle 11 links are in said straightened position, a thrust link interconnecting said plate and said knuckling extension, spring means for biasing said plate to move said knuckling extension to a position for interlocking engagement with said roller to lock said links in said straightening position, and means including a winch having a flexible tensile element connected with said plate for effecting an unlocking limited relative rotation thereof and thereafter applying tensile force to said joint between said links to unstraighten said links and pull said leg and said landing wheel to said retracted position.

REFERENCES CITED The following references are of record in the file of this patent:

' UNITED STATES PATENTS Number Name Date 1,329,533 McCarrol Feb. 3, 1920 1,857,888 Stout May 10, 1932 2,422,577 Martin June 1'7, 194'! FOREIGN PATENTS Number Country Date 408,435 Great Britain Apr, 12, 1943 520,994 Great Britain May 9, 1940 611,733 Germany Apr. 4, 1935 

